1. Field of the Invention
The invention is related to the field of optical particulate measurement instruments, and in particular, to a standard media suspension body, an optical particulate measurement instrument, and a verification method for an optical particulate measurement instrument.
2. Statement of the Problem
A turbidimeter or nephelometer is an instrument used for the measure or study of particles in a suspension media. A nephelometer generally refers to an optical instrument for detecting and/or measuring suspended particulates in a liquid or gas colloid. In contrast, a turbidimeter generally refers to an optical instrument for detecting and/or measuring particulate matter in water. Consequently, the suspension media can comprise water. The suspension media is placed in a sample chamber and light is projected through the suspension media as a beam or cone of admitted light into the suspension media. The particles within the suspension scatter the admitted light by a complex interaction of reflection, diffraction, and refraction. A portion of the light scattered from the particles is received by a detector of the instrument.
The detector is typically positioned at about ninety degrees to the incident light source and a resulting axis of illumination. In order to quantify the amount of particles within the suspension media, a comparison must be made of the received scattered light to a scattered light level obtained using a similar suspension media of known particulate concentration. An instrument that is adjusted or calibrated to read the same as the values of the known suspensions can thereafter be used to determine unknown particulate concentrations. The unknown particulate concentrations can be compared to known calibration values and can be determined by estimation or extrapolation from the calibration values.
FIG. 1 shows a prior art turbidimeter/nephelometer. A light source emits light along the axis of illumination and into a suspension contained within a sample chamber. Light from the light source can propagate through the suspension unimpeded and additionally can interact with the suspension by impinging on particles within the suspension. Light impinging on a particle can potentially scatter in one of multiple ray paths, including a backward scatter ray path, a forward scatter ray path, and can scatter along a ray path substantially at ninety degrees to the incident beam. Light scattered at a right angle impinges on and is quantified by a light receiver. The light receiver converts photon energy into an electrical signal by means of a photoelectric effect. The electrical signal, usually weak or low in signal strength, can be amplified and processed in order to determine a particulate concentration. The particulate concentration can subsequently be output to a meter, display, printout, or other useful indicator.
In order to reduce the cost of unnecessary calibrations, various reusable verification methods and devices have been devised to check the readiness and fitness of use of a turbidimeter or nephelometer. Verification is usually performed after a calibration procedure, wherein verification information is obtained and stored for future verification use. The stored verification information is subsequently compared to instrument readings in the field, whereupon a determination is made as to the need for service by comparing an instrument reading to the stored verification information. Periodic verification is especially critical in applications such as pharmaceuticals manufacturing, food and beverage production, or the production and distribution of potable water.
Current agency regulatory requirements for potable water, such as regulations promulgated by the Environmental Protection Agency (EPA), are drafted to compel and control testing of turbidity. This testing is performed in order to insure the amount of particulate material, pathogens, or parasites in a water distribution system are below a predetermined value or threshold. Commonly, the regulatory requirements dictate that particulates not exceed about one Nephelometric Turbidity Unit (NTU), which greatly reduces the heath risk to the general population. Therefore, periodic testing of the accuracy of turbidity measurement equipment must be conducted, where the turbidity measurement equipment is used to determine the quality of the water. Testing is performed under guidelines set by country, government, and/or state and local municipalities to insure the accuracy of the reported values of turbidity.
Existing methods to validate the compliance of a turbidity system can make use of primary liquid standards, such as suspensions of Formazin and/or liquid suspensions of styrene divinyl benzene polymer beads (SDB or BaS4). Alternatively, compliance testing can use secondary liquid standards such as liquid Latex or solid secondary standards such as acrylics, glass, or glass-ceramics such as SCHOTT ‘Zerodur’ which has an equivalent value as compared to the primary standard. Confidence in the security of drinking water distribution networks is related to the frequency of the validation of the operational readiness and accuracy of the nephelometric measurement systems.
A primary standard can be used for verification, as in U.S. Pat. No. 7,180,594, to Williams. However, usage of such primary standards is expensive, and therefore primary standards are undesirable for frequent or widespread verification.
Alternatively, solid standards have been used in place of liquid standards, as disclosed in U.S. Pat. No. 5,912,737 to Bannerjee or as disclosed in U.S. Pat. No. 5,059,811 to King. However, this prior art approach also has drawbacks. A solid turbidity standard must be precisely aligned and oriented in relationship to both the illuminating beam and the detection means. In addition, the use of solid standards requires service of the operation nephelometric system involving complete removal of the liquid sample and cleaning of all surfaces.
Prior art verification devices and methods generally require that the instrument sample compartment be cleaned and dried before installation of the verification device. Other prior art verification devices and methods require replacement of the sample chamber with a chamber or cell containing a fixed value of turbidity to compare against the last recorded verification.
Consequently, there is a need to empty and clean the test chamber of the turbidity or nephelometry meter, as the test chamber has to be serviced and conditioned before either a liquid test fluid or a solid test master can be used for a verification process. Though cleaning is not required on a nephelometric apparatus that has never been in service, per U.S. Pat. No. 5,757,481 to O'Brien, it does pose considerable effort to routinely bring a field-operational nephelometric system to a condition of clean and dry. Failure to completely remove the sample can result in the formation of a condensate of the sample on a solid standard.
Calibration by use of standard suspension concentrations placed within an empty, dry sample chamber is time consuming and expensive. A new standard suspension is required for each use in order to assure the correct value of the standard and to lessen or minimize the contamination of the standard that can occur if the standard is reused. In addition, prior to calibration, the cleanliness of the instrument must be assured before introduction of the standard into the sample chamber so as not to dilute or alter the results by contamination of the standard suspension. Upon completion of verification, the instrument must again be serviced in order to restore the instrument to an operational condition. The process of restoration or removal of the verification device and can alter or negate the integrity of the instrument readiness or ability to read accurately by improperly performed assembly or restoration.
Failure to align and orient a solid standard precisely or failure to bring the nephelometric system to a state of being properly clean and dry affects the precision of the measurement. This can in turn lead to unnecessary calibration, loss of productivity, or failure to detect a nephelometric system that is out of tolerance or unfit for use. Operation after an improper or incomplete calibration or verification can consequently pose a threat to the integrity of portable water distribution.